This invention relates to coating compositions. More particularly, the invention relates to coating compositions capable of a low temperature cure and especially useful for the coating of sheet molding compounds.
Sheet molding compounds are resinous materials which have found widespread use in recent years. The resinous materials are typically based on polyester resins; however, other types of resins can be used. The resins have become popular due to the fact they are durable and oftentimes as strong as metals. The fact they are also lightweight makes the sheet molding compounds especially useful in the automotive industry. One particular end use has been as a replacement for sheet metal in automobiles.
While parts made from sheet molding compounds do have the requisite durability and strength, it is still necessary that they be primed and top-coated for appearance purposes. A good appearance, though, on such substrates has been difficult to obtain. Surface imperfections or craters in the sheet molding compound substrates act as a trap for solvents contained in conventionally used coating compositions; the trapped solvent which eventually escapes adversely affects the appearance of the substrate. Any coating composition must also have good adhesion to the sheet molding compounds as well as have good hardness, solvent resistance and chip resistance. A coating composition possessing the aforementioned requisites has been difficult to formulate. An additional requirement which would be desirable would be for the coating composition to be capable of curing at a low temperature. Such compositions, of course, have a lessened energy demand.
There has now been found coating compositions which are capable of low temperature cure. The compositions have good hardness, solvent resistance, chip resistance, and the ability to hide substrate non-uniformities. They also have good adhesion to products made from sheet molding compounds. Additionally, the compositions can be formulated as high solids compositions.
As used herein, all percentages and ratios are by weight unless otherwise indicated.